Manufacturers of modern electronic devices, circuits, and systems are able to maintain the quality of their products by the use of inspection steps at a number of stages in the fabrication process. The tools used for such inspections include x-ray inspection systems of various types which are typically classified into two major categories, two-dimensional (2-D) systems and the more recent three-dimensional (3-D) inspection systems. Detailed inspections of areas that are either too small to be seen visually with the unaided eye or are obscured from direct view on printed circuit boards and other electronic articles are often made using such systems. Solder joints on printed circuit boards are of particular interest as these connections often have defects, such as voids, that can negatively impact the reliability of such products.
Two-dimensional systems typically have one area detector that captures and produces a single radiographic image referred to as a 2-D image. An object placed between the x-ray source and the x-ray detector or sensor casts a shadow on the detector and thereby produces an image. Such systems have the advantage of being simple, fast, and relatively inexpensive. However, when more than one objects lies within the x-ray beam, as is often the case with double-sided printed circuit boards, objects on one side of a board and objects on the other side of the board can produce overlapped images. As a result, information important to the inspection can be lost.
Three-dimensional systems use various techniques to capture multiple images of an object and produce images of plane sections through the object referred to as 3-D images. Such techniques are generally referred to as laminagraphic or tomographic techniques. Three-dimensional systems can provide resolution to the problem of overlapping objects. Multiple images of a printed circuit board region are captured at different angles. These multiple images are then processed using tomographic techniques to result in a single image with that single image being an image of a plane section through the object space. Thus, the object imaged may be, for example, from the top, the bottom, or within the printed circuit board.
Laminography is based on the correlated motion of an x-ray source, a detector and an object to be inspected. The x-ray source and the detector are typically moved synchronously in circles 180 degrees out of phase. As a result, the location of the projected images of points within the object moves also. Only points from a particular plane, the so called focal plane, will be projected always at the same location onto the detector and therefore imaged sharply. Object structures above and below the focal plane will be projected at different locations. Because of that, they aren't imaged sharply and will be superimposed as a background intensity to the focal plane.
Digital laminography, or tomosynthesis, is based on the correlated position of the x-ray source and x-ray detector. The source and detector are translated in opposite directions and are positioned at discrete locations when the image is captured. This enables the digital storage of a series of discrete projections which can be subsequently combined.
Linear scan laminography and off axis tomosynthesis is based on the correlated position of the object, the x-ray source, and the x-ray detector. The source and detector are stationary relative one to the other, but move relative to the printed circuit board region. A region is captured at different angles at different times and subsequently combined.